J . Phys. Chem. 1989, 93, 1435-1440
1435
Proton and Deuteron Magnetic Resonance Spectra of Benzene Adsorbed on Alumina and on a Platinum/Alumina Catalyst B. Boddenberg* and B. Beerwerth Lehrstuhl fur Physikalische Chemie II, Universitat Dortmund, Otto-Hahn-Strasse, 0 - 4 6 0 0 Dortmund 50, West Germany (Received: March 23, 1988; In Final Form: August 25, 1988)
The proton ('H) and deuteron (2H) magnetic resonance spectra of a monolayer of benzene on q-alumina and on a platinum/?-alumina catalyst were measured for temperatures in the range 270-75 K. The ZHspectra indicate that below temperatures of about 180 K the liquidlike film becomes transferred into the 2D solid state with molecules rotating rapidly around the hexad axis. From the 'H resonance intermolecular second moments the structure of the low-temperature 2D solid-state layer was determined to be closed-packed hexagonal with unit cell dimension d = 668 5% pm.
*
1. Introduction The highly dispersed defect spinel type y- and q-al~rninas'-~ are of high practical importance because they serve as adsorbents in separation processes," as catalyst^,^ and, most importantly, as support materials for catalysts such as transition metals6.' and mixed oxides.8 In view of this wide range of applications it is not surprising that great efforts have been undertaken to get detailed knowledge on the basic interactions of molecules with the surface of these aluminas in relation to their crystallographic, textural, and chemical properties. In the past, proton ('H) magnetic resonance spectroscopy has contributed a great deal of information in this field of research by giving access to problems of distribution and arrangement of surface OH groups and the proton-exchange dynamics among them as well as the dynamics of molecules adsorbed on such ~urfaces."~ The general advantage of N M R methods over most other spectroscopic techniques of allowing the study of dynamical processes over an extremely wide range of time scale (picoseconds to seconds) is counterbalanced by the disadvantage of 'H N M R that too much information is contained in the spectral data obtained entailing the analysis of the experimental results to be an exceedingly difficult, in many cases even an unsolvable task. This and has the is due to the fact that the proton has spin I = largest gyromagnetic ratio of all stable isotopes causing the dominance of the versatile proton magnetic dipolar couplings over all other internal interaction terms of the nuclear spin Hamiltonian. In recent years, the improved spectrometer technology as well as the development of new N M R techniques has allowed detection of the resonances of nuclei other than the proton, e.g., zH, I3C, and IsN, even in the case of low surface area adsorption systems. Several such publications on studies with aluminas as the solid substrates have appeared.I5-l8 (1) Lippens, B. C. Structure and Texture of Aluminas. Thesis, Technical High School, Delft, Netherlands, 1961. (2) KnBzinger, H.; Ratnasamy, P. Caral. Reu.-Sci. Eng. 1978, 17, 31. (3) Cooke, D. L.; Johnson, E. D.; Merritt, R. P. Catal. Reu.-Sci. Eng. 1984, 26, 163. (4) Lippens, B. C.; Steggerda, J. J. In Physical and Chemical Aspects of Adsorbents and Cafalysrs;Linsen, B. G., Ed.;Academic: New York, 1970; p 171. (5) KnBzinger, H. Ado. Catal. 1976, 25, 184. (6) Burch, R. In Catalysis;Specialist Periodical Report; The Royal Society of Chemistry: London, 1985; Vol. 7, p 149. (7) Taylor, K. In Catalysis. Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1984; Vol. 5, p 119. (8) Ratnasamy, P.; Sivasanker, S. Caral. Reu.-Sci. Eng. 1980, 22, 401. (9) Pfeifer, H. In N M R . Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer: Berlin, 1972; Vol. 7, p 533, and references cited therein. (10) Fripiat, J. J. Caral. Reu.-Sci. Eng. 1971, 5 , 269. ( 1 1 ) Pearson, R. M. J . Catal. 1971, 23, 388. (12) StQbner,B.; Knozinger, H.; Conrad, J.; Fripiat, J. J. J . Phys. Chem. 1978.82, 1811. (13) Cirillo, A. C., Jr.; Dollish, F. R.; Hall, W. K. J . Catal. 1980,62, 379. (14) Tirendi, C. F.; Mills, G. A.; Dybowsky, C. J . Phys. Chem. 1984, 88, 5765. (15) Gottlieb, H. E.; Luz, Z. J . Magn. Reson. 1983, 54, 257.
0022-365418912093-1435$01.50/0
The motivation for undertaking a study of benzene adsorption on q-alumina and on a Pt/q-alumina catalyst was 2-fold. First, it was of interest to get knowledge on the motional behavior of an adsorbed molecule which is highly symmetric and can be considered to be rather inert with respect to the interaction with the oxide, and at the same time to check the unusual results obtained in ref 15 with the aid of a well-defined alumina as the substrate. Second, with the application of both proton ('H) and deuteron (2H) N M R spectroscopy to the same system, it was intended to examine the possibility of disentangling the involved proton resonance results obtained for such adsorption systems. For this purpose the 2H nucleus is very well suited since the interaction of the nuclear quadrupole moment with the electric field gradient tensor at the deuteron site represents the by far dominating internal spin Hamiltonian. This entails that via the 2H results the intramolecular contributions to the proton resonance data can readily be assessed and quantitatively taken into account. In the present paper the information content of the IH and *H N M R spectra is analyzed whereas in the subsequent paperI9 the 'H and 2H relaxation behavior is the subject of concern.
2. Experimental Section 2.1. Preparation of q-A1203and Pt/q-A1203Catalyst. The q-alumina used for the present investigation was prepared from aluminum metal foil (Vereinigte Aluminiumwerke, Bonn; 99.99% AI with Si (15 ppm), Fe, Cu, Mg (6 ppm each), and Ca (3 ppm) as the main impurities) by hydrolysis to Al(OH)3 (Bayerite) according to Schmah's method'*20and subsequent dehydration of the hydroxide at 230 OC for about 24 h in air. The dehydration curve of the oxide which is depicted in Figure 1 reveals that considerable amounts of water are expelled by heating up to 1200 "C. The loss of water is referred to the weight of the oxide at this latter temperature. By X-ray diffraction the hydroxide and the oxide at calcination temperatures between 230 and 800 OC were identified as rather well crystallized Bayerite and q-Alz03, respectively, with the latter exhibiting crystallite sizes of about 8 nm as estimated from the line broadenings. An ambient temperature X-band ESR analysis of the Bayerite yielded a concentration of 1.4 X 10I6spins/g AI(OH), of not clearly identifiable nature. The 3.4% Pt catalyst was prepared from this q-A1203by impregnation with aqueous solutions of H2PtC16.6H20 (Baker, Gross-Gerau, Germany). After drying at several selected temperatures up to 200 OC in helium flow the material was reduced with streaming hydrogen at 300 OC for 3 h and subsequently flushed with dry helium at 300 OC (1 h), at 160 "C (14 h), and (16) Majors, P. D.; Raidy, T. E.; Ellis, P. D. J. Am. Chem. Soc. 1986, 108, 8123. (17) Majors, P. D.; Ellis, P. D. J . Am. Chem. SOC.1987, 109, 1648. (18) Boddenberg, B. In Lectures on Surface Science; Castro, G. R., Cardona, M., Eds.; Springer: Berlin, 1987; p 226. (19) Boddenberg, B.; Beerwerth, B. J. Phys. Chem., following paper in this issue. (20) Schmah, H. Z . Naturforsch. 1946, 1, 323.
0 1989 American Chemical Society
Boddenberg and Beerwerth
1436 The Journal of Physical Chemistry, Vol. 93, No. 4,1989
b
1 5 t
200
600
LOO
ICE
800
1200
I
1
Figure 1. Dehydration curve for calcining in air of q-alumina prepared ,
1
1
1
l
l
l
l
i
n S / mmoi
11
10
9
0
7
6
5
1
PI Po 01
02
03
OL
05
06
07
08
09
at I1 K .
during cooling down to ambient temperature. The X-ray diffraction analysis of the material that had turned gray after this preparation procedure reproduced the q-A1203lines but gave no perceptible Pt metal lines indicating the Pt crystallite sizes to be below 3 nm. 2.2. Texture of q-A1203and Pt/q-A1203 Catalyst. Figure 2 shows the low-temperature (77.7 K) nitrogen adsorption and desorption isotherms of q-A1203calcined at 750 "C for 32 h in vacuo. From the BET plot (Figure 3a) being linear in the pressure range 0.05 < p / p o < 0.4 the specific surface area was determined as SBEr = 220 m2/g using the standard nitrogen molecule cross section w N 2= 0.162 nm2.21The pore size distribution analysis,21 based on the slit shaped pore geometry model as suggested by the form of the hysteresis loop (Figure 2), yielded the distribution function depicted in Figure 3b having the center at 2.9 nm and (21) Gregg, S.I.; Sing, K.S . W. Adsorption, Surface Area and Porosity; Academic: London. 1967.
Figure 3. BET plot (a) and pore size distribution function (b) from the isotherms of Figure 2.
a rather narrow width. Once heated in vacuo at 750 OC for at least 24 h, the q-alumina exhibited practically the same texture data irrespective of subsequent rehydroxylation and heating to 400 OC as well as of treatments with oxygen at 400 "C and with hydrogen at 300 "C. At 800 "C calcining temperature the specific surface area decreased to about 170-140 m2/g depending mainly on the calcining time between 16 and 50 h indicating the onset of sintering. The texture properties of the P t / ~ - A 1 , 0catalyst ~ turned out to be similar to those of the support from which they were prepared. 2.3. Preparation of NMR Samples. q-Al2O3 (1 70 m2/g) calcined at 800 "C and the Pt catalyst prepared from the 800 OC calcined oxide served as starting materials for the preparation of the N M R samples. After these materials were put into 10 mm outside diameter glass tubes, evacuation at 400 "C was followed by three 400 OC oxygen contact (300 mbar of 02,'/2 h)/evacuation cycles, a 300 "C hydrogen contact (30 mbar of H2, h)/evacuation procedure, and finally rehydroxylation at room temperature via H 2 0 or D 2 0 gas-phase contacts with a subsequent 16-h evacuation at 400 OC. It is expected that after this procedure the oxide support exhibits well-defined texture and hydroxylation characteristics irrespective of the presence of the metal. The OH-group density on the q-A1203support prepared as described before was estimated from 'H N M R signal intensity measurements to be 6 f 1 OH/nm2, which value is comparable with data reported in the literature.2 Expressed in equivalents of water this figure corresponds to 15 f 2.5 mg of H 2 0 / g of A1203and is, thus, considerably lower than the data obtained for the oxide calcined in air (Figure 1). The loadings of the samples carrying surface O H and OD groups with, respectively, benzene-d, and benzene-h6 (both Merck, Darmstadt, Germany) were performed by chilling calibrated amounts from the gas phase onto the solids to give one-monolayer coverages using 0.40 nm2 as the benzene cross section area.2' For use in the spectrometer the sample containers were sealed off. 2.4. NMR Techniques,, The N M R measurements were performed with the aid of a high-power pulsed FT-NMR spectrometer (CXP 100, Bruker-Physik, Karlsruhe, Germany) operated at the resonance frequencies w0/27r = 13.7 and 89.1 MHz (2H and 'H; 2.1-T field) and at 52.7 MHz (2H;8.3-T field). Home-built probe heads and on-line computer facilities were used. Temperature regulation and control in the range 300-77 K was achieved to fO.l and f 2 OC when static cryostate and dynamic gas stream operations were applied, respectively. The first mentioned operation mode was used with measurements in the cryogenic magnet system. In the proton resonance case the free induction decays (FID) after ~ / 2 - p u l s eexcitation were detected. It was observed that at low temperatures ( T < 120 K) the short-time FID shape could well be approximated by a Gaussian allowing the second moment (M2,e,pJof the spectrum to be obtained within an estimated error of 110%. The well-known relation
N M R of Benzene Adsorbed on 7-A1203 and Pt/q-AlZO3
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1437
I 230 K
I
162 K
I
208 K
I
190 K
175 K
135 Y
Figure 4. Deuteron spectra of one monolayer of benzene-d6 on ?-alumina. The theoretical spectrum is based on DQCC = -93 kHz and bv = 1 kHz
Gaussian line broadening.
I
271 K
I
256 K
208 K
230 K
I
I
190 K
I
162 K
95 K
throrrtlerl
Figure 5. Deuteron spectra of one monolayer of benzene-d, on Pt/T-Al20, catalyst. The theoretical spectrum is based on DQCC = -93 I
kHz and bv
1 k U 7 f h i i r r i a n line hrnarlenino
was applied where A ( t ) represents the FID shape. The deuteron resonance spectra were obtained by Fourier transformation of the FID's or of the quadrupole echoes generated by the quadrupole echo pulse sequenceZZand detected in quadrature in cases where motionally averaged singlet or solid powder pattern type spectra came out, respectively. At resonance frequency 13.7 MHz the quadrupole echoes to be Fourier transformed were coherently added signals following the alternately applied sequences (?r / 2 ) x - ~ - (A / 2),-recho T / ~ ) ~ - T -T( / 2),-~-echo (2) By this procedure the severe acoustic ringing effects were eliminatedeZ3 Not unexpectedly, the solid-state patterns thus obtained were practically identical with those measured at the resonance frequency 52.7 MHz. (T),-t-(
(22) Davis, J. H.; Jeffrey, K.R.; Valic, M.I.; Higgs, T.P. Chem. Phys. Lett. 1976, 42, 390.
(23) Neue, G. Dissertation, Universitat Dortmund, 1983.
3. Results Figures 4 and 5 show the 2H N M R spectra for one monolayer of benzene-d6 adsorbed on 7-Al,03 and Pt/7-AIz03, respectively, at several selected temperatures between 270 and 95 K. It may be recognized that, besides a systematic shift by about 2 0 K to higher temperatures, the spectra of the loaded catalyst exhibit practically the same sequence of spectral shapes as the loaded oxide. This sequence consists of Lorentzian-type singlets of increasing widths which after crossing some transition range become completely transformed into temperature-independent solid-state powder patterns of the Pake type with prominent edge splitting Av = 7 0 kHz and outer edge separation of 140 kHz. The pure solid state pattern shape is fully developed at about 130 and 150 K for the oxide and the catalyst, respectively, and pertains down to 80 K in both cases. The transition-range spectra essentially consist of superimposed Lorentzians and solid-state patterns of 70-kHz splitting where the latter increase in intensity at the expense of the singlets. From the low-temperature (120-80 K) 'H free induction decays temperature-independent second moments M2,cxptl = (9 f 1) X
1438 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Boddenberg and Beerwerth
1 Os s-’ were determined for both the benzene/alumina and the benzene/catalyst systems.
4. Discussion
4.1. Low-Temperature Solid-state Spectra. 4.1 .I. 2H Spectra. The Pake-type shape of the low-temperature powder patterns (Figures 4 and 5) immediately reveals axial symmetry (7 = 0) of the EFG tensor being operative. From the Au = 70 f 1 kHz edge splitting the deuterium quadrupole coupling constant (DQCC) is readily calculated as24 IDQCCexpI= 4 / 3 Av = 93.3 f 1.3 kHz, which is half of the rigid DQCC = e2qQ/h within the range of DQCC values (180.7-196.5 kHz) reported in the litera t ~ r e . ’ ~ , ’This ~ reduction in splitting is undoubtedly due mainly to hexad axis rotation proceeding fast on the N M R time scale T N M R = ($qQ/h)-’ = 3.4 ks since the coupling constant for rapid single-axis rotation is DQCC = y2(3 cos2 A
- I)S(e’qQ/h)
(3)
yielding 3 cosz A - 1 = -1 with A = 7r/2 (A is the angle between the EFG tensor distinct principle axis, Le., the C-D bond direction, and the axis of rotation). In eq 3 a fast wobbling type of motion of the rotation axis has been taken into account by the order parameter” S = 72(3 COS’ 6 - 1)
(4)
where B is the angle between the instantaneous and the mean orientation of the hexad axis, and the bar denotes the average over the wobbling motion. S (0 IS I1) has been introduced to account for the surprisingly large discrepancies of the e2qQ/h values quoted before. Before drawing conclusions about the value of S and, hence, about e2qQ/h derived from the present experiments, it is worthwhile to realize the very sharply developed contours of the powder patterns from which the individual line widths can be estimated to be about 1 kHz only. Actually, such line widths can already be accounted for by the inhomogeneity of the static Zeeman field applied which for the present purposes was not set to high-resolution conditions. It follows that the natural individual line widths may be even smaller than 1 kHz. This is a very surprising result because for benzene adsorbed on various solid substrates, e.g., zeolite^^^-^^ and graphitized c a r b ~ n s , ~as l - well ~ ~ as for powder samples of crystalline benzene34 considerably broader lines are obtained. In general, the broadening may arise from magnetic couplings of the deuterons with other surrounding spins but also by some distribution of the order parameter S . The latter effect would have been expected to be operative for the presently studied benzene/alumina systems because of the geometric and energetic surface heterogeneities usually encountered with aluminas. Obviously, the present results lead to rejection of this notion. Since it is hardly conceivable that the adsorbed benzene molecules on alumina exhibit values S = 1 with, at the same time, very low standard deviation, it is concluded that S = 1; Le., the hexad axis of each adsorbed benzene molecule is almost perfectly oriented in space. This conclusion has two interesting consequences. First, putting S = 1 in e q 3 gives e 2 q Q / h = 186.6 rf 2.5 kHz in excellent (24) Rinne, M.; Depireux, J. Ado. Nucl. Quadrupole Reson. 1974, 1, 357. (25) Millett, F. S.; Daily, B. P. J . Chem. Phys. 1972, 56, 3249. (26) Barnes, R. G. Adu. Nucl. Quadrupole Reson. 1974, 1, 335. (27) Wittebort, R. J.; Szabo, A. J . Chem. Phys. 1978, 69, 1722. (28) Hasha, D. L.; Miner, V. W.; Garces, J. M.; Rocke, S . C. In Catalyst Characterization Science; Deviney, M. L., Gland, J. L.,Eds.; ACS Symposium Series 288; American Chemical Society: Washington, DC, 1985; p 485. (29) Eckman, R. R.; Vega, A. J. J . Phys. Chem. 1986, 90, 4679. (30) Boddenberg, B.; Burmeister, R. Zeolites, in press. (3 I ) Boddenberg, B.; Grosse, R.; Horstmann, W.; Neue, G. Colloids Surf. 1984, 11, 265. (32) Boddenberg, B.; Grosse, R. 2.Naturforsch. 1986, 410, 1361. (33) Grosse, R.; Boddenberg, B. 2.Phys. Chem. (Munich) 1987, IS2, 1 . (34) Hentschel, R.; Schlitter, J.; Sillescu, H.; Spies, H. W. J . Chem. Phys. 1978, 68, 56
Figure 6. Arrangement of benzene molecules in the low-temperature 2D solid state. The 2D unit cell constant is d = 668 pm.
agreement with the most reliable value of the rigid quadrupole coupling constant as derived from N M R single-crystal meas ~ r e m e n t . ~ ~Second, this result implies that-not unexpectedly-the intramolecular benzene molecule charge distribution is not affected by the bonding to the surface as far as it influences the EFG tensor at the deuteron sites. It might be conjectured that the experimental results originate from 3D solid benzene crystallites which have formed somewhere in the sample container as a consequence of desorption from the surface and subsequent recrystallization. This possibility can, however, be discarded because at temperatures below about 100 K the hexad axis rotation correlation time in solid benzene has risen to values far above 7NMR.35’36 Consequently, a Pake pattern of splitting Av = 140 kHz then should have appeared as is the case with, e.g., a 10-monolayer benzene film on graphite.36 In the present case such a situation definitely does not show up. 4.1.2. Proton Resonance Second Moments. The proton resonance second moment for the fully protonated benzene molecules adsorbed on the deuteriated 7-Alz03surface may be written as M2
=
M2,intra
+ M2,inter + MZ.H-AI + M2,H-e
(5)
where the first two terms refer to the homonuclear intra- and intermolecular dipolar couplings and the latter terms take into account the proton couplings with the aluminum spins and with the unpaired electron spins of the impurity centers at or near the support surface, respectively. Because of the low concentration of impurities present in the oxide, the last term of eq 5 may fairly be neglected. Taking the (1 11) layer of alumina at 50% hydroxylationZas a model with ’ assuming the benzene molecules oxide ion radius 0.13 n ~ n , ~and (van der Waals thickness 0.34 nm38) adsorbed flat, the rigid MZ.H-AI second moment39is calculated to be negligible in comparison to Mz,exptl. With the knowledge of rapid hexad axis rotation from 2H NMR and, therefore, MZbtia= 5 . 6 X lo8 s - ~(ref 35), the intermolecular = 3.4 X lo8 s-’ using the second moment is obtained as M2,inter cited previously. This experimentally determined value M2,cxptl value of M2,inter which amounts to about 60% of M2,intra as well of the three-dimensionally densely packed benzene as of Mz,inter crystal3s suggests a rather closely packed arrangement of the benzene molecules in the two-dimensionally spread adsorbed layer. The most efficient two-dimensional close packing of benzene molecules with shortest possible intermolecular proton-proton distances is obtained with the hexagonal arrangement depicted in Figure 6. In fact, for benzene adsorbed on the basal planes of both mono- and polycrystalline graphite such an arrangement has been proven experimentally to exist at temperatures below (35) Andrew, E. R.; Eades, R. G. Proc. R. Soc. London, A 1953,218, 537. (36) Boddenberg, B.; Grosse, R. 2. Naturforsch. 1987, 42a, 272. (37) Handbook of Physics and Chemistry, 67th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1987. (38) Stair, R. C.; Somorjai, G. A . J . Chem. Phys. 1977, 67, 4361. (39) Abragam, A. The Principles of Nuclear Magnetism; Clarendon: Oxford, 1961.
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1439
N M R of Benzene Adsorbed on 7-A1203and Pt/q-Al,O, about 130 K with the benzene layer being in registry with the graphite lattice.w2 In addition, on graphite the benzene molecules are in rapid hexad axis motion32 as they are in the presently studied systems. In analogy, an arrangement of the benzene molecules as in Figure 6 is suggested to be prevailing in the present cases, too. In order to verify this notion, the rapid hexad axis motion of the benzene molecules is considered to consist of f60° jump-type reorientations with centers of gravity remaining fixed on the lattice points. For such a situation the calculation of the intermolecular second moment for a pair of spins located on different molecules has been worked out p r e v i ~ u s l y . ~Using ~ the relevant formula43 and taking into account the high symmetry of the problem the intermolecular second moment is
In eq 7 ?kokl and are vectors connecting a proton in positions ko,loof a reference molecule (0) with a proton in positions klJ1 on a neighboring molecule (j), and rbk1 and rlol are the corresponding moduli. The indices ko,lo and ki,lieach run from 1 to 6, i.e., over the orientational positions of the molecules denoted by 0 and j , respectively. Each term DMJldlis proportional to where d is the lattice constant of the two-dimensional unit cell can sensitively be fitted by variation (Figure 6). Thus, Mz,inter of d. Regarding all molecules within the first three coordination circles of any reference molecule containing six molecules each, the computer calculation of eq 6 yields d = 668 pm f 5% to attain = 3.4 X l o s f 30%. This value of coincidence with MZinter(cxptl) d corresponds to a distance of closest approach between protons of neighboring molecules of 253 f 12 pm which practically is in coincidence with the intramolecular proton-proton distance (249.5
e,
Most surprisingly, the most probable value d = 668 pm excellently agrees with the unit cell constant of a 2D solid benzene monolayer on graphite as deduced from a LEED study using a graphite single-crystal surface.42 In this study it was also found that the ordered 2D solid benzene phase is in registry with the underlying graphite (0001) crystal plane. In the light of the present study this result appears to be fortuitous since with alumina and the Pt/alumina catalyst where no such highly symmetrical support lattice is available practically the same lattice constant is obtained. This suggests that the van der Waals interaction between the molecules mainly determines the arrangement of the molecules adsorbed on the solid surfaces considered here. 4.2. Transition and High-Temperature-Range 2H Spectra. In the subsequent paper19 it will be shown that the collapse of the low-temperature powder patterns into the high-temperature Lorentzian-type singlets (Figures 4 and 5 ) is caused by molecular translational diffusion across the randomly oriented surfaces of the adsorbent particles. Thus, it should be expected that the typical transition spectrum shapes45appear when the translational correlation T~ becomes the order of T N M R introduced previously. Apparently, such a behavior does not show up presently but the spectra seem to reveal a separation into two phases exhibiting the (40) Monkenbusch, M.; Stockrneyer, R. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 808. (41) Meehan, R.; Rayrnent, T.; Thomas, R. K.; Bomchil, G.; White, J. W. J . Chem. SOC.,Faraday Trans. I 1980, 76, 2011. (42) Bardi, U.; Magnanelli, S.; Rovida, G.Surf. Sci. 1986, 165, L7. (43) Boddenberg, B.; Moreno, J. A. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 83. (44) Boddenberg, B.; Moreno, J. A. J . Mugn. Reson. 1978, 29, 91. (45) Smith, I. C. P. In N M R of Newly Accessible Nuclei; Laszlo, P., Ed.; Academic: New York, 1983; Vol. 2.
low- and high-temperature characteristics of powder pattern (Av = 70 kHz) and Lorentzian singlet shapes, respectively. Most probably, this behavior is due to a phase transition from a 2D fluid state into the low-temperature 2D ordered solid state. Such phase transitions are commonly observed for molecular adsorption on substrates with well-defined surface crystallography, e.g., graphite and boron nitride.&v4' For benzene on graphite this phase transition occurs in some range of temperature at about 130 K.40342The present results give some evidence that phase transitions of the 2D fluid/solid type are feasible also on crystallographically much less well-defined surfaces. It should, however, be noted that according to the idealizing model of Knozinger and RatnasamyZconcerning the distribution and arrangement of surface hydroxyls on partly dehydroxylated y- and 7-aluminas crystallographically well defined surface planes could be established. According to these authors such a situation could especially be realized with 50% hydroxylation (6.5 OH groups/nm2 for the ( 1 11) plane), which is about the O H group density that was adjusted with the preparation of the presently used samples. It is interesting to notice that the temperature range in which the benzene 2D phase transition occurs on the homogeneous graphite surface is shifted by at least 20-30 "C to lower temperature as compared with the presently studied alumina and Pt/alumina supports. An explanation for this finding is hardly to be arrived at presently. Apparently, the characteristics of the molecule/surface bond play a role. The high-temperature Lorentzian-type 2H N M R lines will be considered in detail in the context of the relaxation behavior of the adsorbed molecule^.'^ Most interestingly, however, the presently found Lorentzian shaped lines are drastically different from the powder pattern type spectra reported in the l i t e r a t ~ r e ' ~ for the benzenealumina system. There seems to be a very simple explanation for these discrepancies. The presently used alumina is mesoporous according to the pore size analysis result reported in section 2. Hence, it is the interaction of the molecules with the freely accessible surface that causes the Lorentzian lines. This statement will be substantiated in ref 19. The reason simply is that an isotropic averaging comes about through the rapid translational motions of the adsorbed molecules along the particle surfaces. Should the support material, however, be microporous, thus having pore sizes of molecular dimensions, an orientation of the molecules dictated by the pore geometry would result. Such a situation seems to be prevailing with the alumina used in ref 15. Unfortunately, this notion cannot be verified because of the lack of relevant information. 5. Conclusions
It was the aim of the present work to show that, with the combination of proton and deuteron magnetic resonance, information about the structure as well as the molecular dynamics of a one-monolayer film of benzene on alumina and a Pt/alumina catalyst at low temperatures can be obtained. The shapes of the 2H powder patterns unequivocally reveal the rapid hexad rotation motion of the adsorbed molecules whereas the 'H second moments give evidence for a closed-packed arrangement of the molecules in the layer. For this latter conclusion to arrive at the information from 2H NMR about the rotational type of molecular motion proceeding is of basic importance. The present results suggest that below about 150 K on both the alumina and catalyst supports the benzene monolayer exists in a 2D solid state of structure and dynamics similar to the phases that have been detected with various techniques to exist on graphite. Some evidence has been obtained that a 2D fluid/solid-type phase transition is operative as in the case of the homogeneous substrates. It seems probable that the van der Waals interactions among the adsorbed molecules and not the adsorption forces to the surface are mainly responsible for the occurrence of a phase (46) Phase Transitions in Surface Films; Dash, J . G . , Ruvalds, J., Eds.; Plenum: New York, 1980. (47) Ordering in Two Dimemiom; Sinha, S . K., Ed.; North-Holland: New York, 1980.
1440
J. Phys. Chem. 1989, 93, 1440-1447
transition and the building up of a closed-packed 2D solid phase. To test this general conclusion, measurements are being undertaken with other adsorbents as substrates.
forming the computer calculations. Financial support of this work by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie is gratefully acknowledged.
Acknowledgment. We thank DipLChem. G. Auer for per-
Registry No. AI2O3, 1344-28-1; Pt, 7440-06-4; benzene, 71-43-2.
Proton and Deuteron Magnetic Resonance Relaxation of Benzene Adsorbed on Alumina and on a Platinum/Alumina Catalyst B. Boddenberg* and B. Beerwerth Lehrstuhl f u r Physikalische Chemie II, Universitat Dortmund, Otto- Hahn-Strasse, 0 - 4 6 0 0 Dortmund 50, West Germany (Received: March 23, 1988; In Final Form: June 28, 1988)
The proton ('H) and deuteron (2H) magnetic resonance relaxation times T I and T2of one monolayer of benzene on g-alumina and a platinum/g-alumina catalyst were measured at two Zeeman field strengths (2.1 and 8.3 T) as function of temperature in the range 270-160 K. A model for the motions of the adsorbed benzene molecules is developed that allows an almost quantitative treatment of the experimental relaxation data. By taking into account the intramolecular contribution to the 'H relaxation rates through the results obtained from 'H relaxation, the interpretation of the proton data is made feasible. From both the IH and 2H relaxation times surface diffusion coefficients are derived which compare rather well with respect to both absolute values and temperature dependence
1. Introduction
It is well-known'-3 that the spin-lattice ( T I )and spin-spin (T2) relaxation times of protons contained in molecules adsorbed on the surfaces of solids most often are strongly influenced or even dominated by the dipolar couplings with paramagnetic impurity centers of the support unless the concentration of such species is lower than the order of 10 ppm. This circumstance prevents unambiguous information about the dynamics of adsorbed molecules to be obtained with adsorbents and catalysts of practical interest. In addition, the versatile magnetic dipolar couplings of the protons with spins other than electronic such as the protons being contained in the same and the other molecules as well as in surface O H groups tremendously increase the problem of interpreting appropriately the experimental data obtained. One way to get out of this dilemma was pointed out several years ago by Pfeifer and co-workers,' who introduced the method of relaxation analysis which systematically uses the technique of substitution and dilution of the proton spin system with deuterium. This rather tedious method which, in principle, fully exhausts the information content of the proton relaxation data requires, however, low statistical error N M R data and most accurately reproducibly prepared samples. In the present paper it is proposed to use deuteron (2H) relaxation as the main source of information about the molecular dynamics of the adsorbed species. As far as present experience goes, the interaction of the 2H nuclear quadrupole moment with the electric field gradient (EFG) at the nuclear site is practically the only source for relaxation, thus getting rid of the most complicated dipolar effects. Since the EFG tensor is predominantly intramolecular in origin and, hence, has fixed orientation with respect to the molecular framework, the 2H relaxation data purely reflect the reorientational types of molecular motion. Since, however, the translational motions along the surface of the particulate adsorbents correspond to molecular reorientations as well," the evaluation of surface diffusion Coefficientsby 2H NMR should be feasible. (1) Pfeifer, H. In NMR. Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer: Berlin, 1972; Vol. 7, p 53. (2) Resing, H. A. J . Chem. Phys. 1967, 46, 4701. (3) Boddenberg, B.; Moreno, J. A Ber. Bunsen-Ges. Phys. Chem. 1983,
87, 83 (4) Grosse, R.; Boddenberg, B. Z. Phys. Chem. (Munich) 1987, 152, 1.
0022-3654/89/2093- 1440$01.50/0
Finally, with the knowledge of the reorientational dynamics of the adsorbed molecules from 2H NMR, the intramolecular contribution to 'H N M R relaxation can be calculated and taken into account, whence leaving an interpretatory problem of reduced complexity. 2. Experimental Section
The preparation and characterization of the samples used for the present investigation have been described previ~usly.~For comparison purposes two samples of g-A1203were prepared where the first (sample A) was treated as described5 and the second (B) was not reduced with H2 in the final preparation step. The 'H and 2H relaxation measurements were performed at resonance frequencies w 0 / 2 s equal to 89.1 ('H), 52.7 (2H), and 13.7 MHz (2H) by using the equipment described e l s e ~ h e r e .The ~ spin-spin relaxation times T2were determined from the heights of the spin echoes produced by Hahn and CPMG pulse sequences6 in the appropriate temperature ranges (see Figure 2) in the case of 'H resonance and from the exponentially decaying free induction decays in the case of 2H resonance. The spin-lattice relaxation times TI were obtained with the aid of saturation and inversion recovery pulse sequences6in cases where the conditions T2
